This invention relates to the heat treatment of steel articles, and in particular relates to induction heating, quenching, and tempering of steel sheets.
In order to improve the mechanical properties of metal articles, metal is typically subjected to time consuming, and therefore costly, heat treatment processes. To increase the hardness of a steel, a steel article may be subjected to a heating cycle at or above a temperature of the metal's critical temperature, followed by quenching the metal article. This process typically results in creation of a martensitic microstructure in steels. Martensitic microstructures, while very hard, are also known to be relatively brittle, i.e., having little ductility. To increase the ductility of martensitic microstructures, such steels are often tempered, or heated to a temperature below the steel's critical temperature, whereby stresses built up in the steel during quenching are reduced. Such heating, quenching, and tempering processes are typically long to conduct, and accordingly, expensive.
In processing steel generally, and, more specifically, in forming anti-ballistic armor, it has until now been difficult to achieve a metal product having a combination of strength and ductility which could be manufactured without high cost, including extensive heat treatment time. For example, such a metal article should be able to resist penetration by armor piercing ammunition as well as fragments from improvised explosive devices, including explosively formed projectiles. We have found a method and apparatus for heat treating, quenching, and tempering a steel article whereby the article has desirable mechanical and microstructure properties, including properties which may be useful in acting as anti-ballistic armor or in other applications which may require a steel sheet having high hardness in combination with high ductility.
Disclosed is a method for treating a steel article to form a high hardness and high ductility alloy comprising the steps of:
(a) providing a steel composition having a material thickness less than 0.5 inches (12.7 mm), having an initial microstructure of ferrite and pearlite, and having a composition of, by weight,
carbon between 0.25 and 0.55%.
silicon between 0.15 and 0.35%,
manganese between 0.40 and 1.0%,
chromium between 0.80 and 1.10%,
molybdenum between 0.15 and 0.25%,
sulfur less than 0.040%,
phosphorus less than 0.035%;
(b) heating the provided steel composition to a peak temperature of between 850° C. (1562° F.) and 1150° C. (2102° F.) in less than ten seconds;
(c) holding the heated steel composition at a temperature within the peak temperature range for between two and ten seconds;
(d) quenching the heated steel composition from the peak temperature range to below 100° C. (212° F.) at a temperature rate reduction of between 400 and 3000° C./sec (752-5432° F./sec);
(e) removing residual quench media from the surface of the quenched steel composition;
(f) tempering the quenched steel composition at a temperature from 100° C. to 260° C. (212-500° F.) for less than ninety minutes;
(g) air cooling the tempered steel composition to less than 100° C. (212° F.) to form a steel article having a transformed microstructure at least 80% martensite and up to 5% bainite, a yield strength of at least 1800 MPa, a total elongation between 5% and 12%, and having a V50 protection ballistic limit at 30° obliquity angle between 2200 and 2700 feet per second (670-823 m/s) with a .30 caliber armor piercing round for a thickness of 0.25″ (6.35 mm)
In the disclosed method, the steel may be heated in the heating step in less than five seconds or alternatively in less than four seconds. Additionally, the heating step may be performed using an induction heater. Following the heating step, the heated steel composition may alternatively be held at the peak temperature range for between two and six seconds.
Alternatively, disclosed is a method for treating a steel article to form a high hardness and high ductility alloy comprising the steps of:
(a) providing a steel composition having a material thickness less than 0.5 inches (12.7 mm), having an initial microstructure of ferrite and pearlite, and having a composition of, by weight,
carbon between 0.25 and 0.55%.
silicon between 0.15 and 0.35%,
manganese between 0.40 and 1.0%,
chromium between 0.80 and 1.10%,
molybdenum between 0.15 and 0.25%,
sulfur less than 0.040%,
phosphorus less than 0.035%;
(b) heating the provided steel composition to a peak temperature of between 850° C. (1562° F.) and 1150° C. (2102° F.) in less than ten seconds;
(c) holding the heated steel composition at a temperature within the peak temperature range for between two and sixty seconds;
(d) quenching the heated steel composition from the peak temperature range to below 100° C. (212° F.) at a temperature rate reduction of between 400 and 3000° C./sec (752-5432° F./sec);
(e) removing residual quench media from the surface of the quenched steel composition;
(f) tempering the quenched steel composition at a temperature from 100° C. to 260° C. (212-500° F.) for less than ninety minutes;
(g) air cooling the tempered steel composition to less than 100° C. (212° F.) to form a steel article having a transformed microstructure at least 80% martensite and up to 5% bainite, a yield strength of at least 1800 MPa, a total elongation between 5% and 12%, and having a V50 protection ballistic limit at 30° obliquity angle between 2200 and 2700 feet per second (670-823 m/s) with a .30 caliber armor piercing round for a thickness of 0.25″ (6.35 mm)
The steel composition may be preheated at least 2.2° C. (35.9° F.) per second to not more than 815° C. (1500° F.) before heating in step (b). Alternatively, the steel composition may be preheated to a temperature between 260° C. (500° F.) and 538° C. (1000° F.) and then preheated at least 0.7° C. (33.3° F.) per second to not more than 815° C. (1500° F.) before step (b).
Additionally, prior to heating the steel composition, two or more lengths of steel plates may be welded together along the width with one or more welds to form a continuous series of steel plates. Further, the step of welding may include applying a weave weld bridging between lengths of steel plate across the width of the steel plates. Further, the step of welding may include applying a weave weld bridging between lengths of steel plate in three sections where the center portion of steel plate is done first and the side portions are welded to provide a weave weld across the width of the steel plates. In any event, a seam weld is applied over the weave weld across the width of the steel plates. Further, an indicia may be applied to the steel plate in advance of the welding step to enable a vision system to identify the location of end portions of lengths of the steel plates for the welding step.
During the quenching step, the heated steel composition may be quenched from the peak temperature range to below 50° C. (122° F.) at a temperature rate reduction of between 400 and 3000° C./second (752-5432° F./sec). In the disclosed method, the quenching step may be performed by, among the suitable quenching methods, flowing a quench medium over the steel article at a rate of up to 900 gallons/min (3400 L/min). In one alternative, the quench medium may be water. Following quenching, the residual quench media may be removed from the surface of the quenched steel composition by at least one of mechanical wiping, blown air, and combinations thereof. The quenching step may be, for example, performed in more than 1 second and not more than 20 seconds.
The quenched steel composition may be tempered using an induction heater for less than 30 minutes. The quenched steel composition may also be oven tempered for less than ninety minutes. The quenched steel composition may also be tempered by a combination of oven and induction tempering for 30 to 90 minutes. In still yet another alternative, the quenched steel composition may be induction tempered for two minutes or less. The tempering step may be performed at between 120° C. (250° F.) and 400° C. (750° F.) in a time between 1 and 10 seconds. After quenching or tempering step, the steel plate may be cut into lengths at least at the seams to make substantially rectangular processed steel product while the steel plate continuously moves along the conveyor.
Also disclosed is a method for treating a steel article to form a high strength and high ductility alloy comprising the steps of:
(a) providing a steel composition having a material thickness less than 0.5 inch (12.7 mm), having an initial microstructure of ferrite and pearlite, and having a composition of, by weight:
carbon between 0.25 and 0.55%,
silicon between 0.15 and 0.35%,
manganese between 0.40 and 0.60%,
chromium between 0.80 and 1.10%,
molybdenum between 0.15 and 0.25%,
sulfur less than 0.040%,
phosphorus less than 0.035%;
(b) heating the provided steel composition to a temperature of 850-1150° C. (1562-2102° F.) in less than ten seconds;
(c) holding the heated steel composition at the peak temperature range for between two and ten seconds;
(d) quenching the heated steel composition to below 100° C. (212° F.) in less than twenty seconds;
(e) removing residual quench media from the surface of the quenched steel composition by at least one of mechanical wiping, blown air, and combinations thereof;
(f) tempering the quenched steel composition at a temperature of; a range from 100° C. to 260° C. (212-500° F.) for less than 90 minutes;
(g) air cooling the tempered steel composition to less than 100° C. (212° F.) having a transformed microstructure of at least 80% martensite and up to 5% bainite, a yield strength of at least 1800 MPa, and a total elongation between 5% and 12%.
The steel composition may be heated in the heating step in less than eight seconds or alternatively in less than six seconds. Again, the heating step may be performed using an induction heater. Again following the heating step, the heated steel composition may alternatively be held at the peak temperature range for between two and six seconds.
Alternatively, disclosed is a method for treating a steel article to form a high strength and high ductility alloy comprising the steps of:
(a) providing a steel composition having a material thickness less than 0.5 inch (12.7 mm), having an initial microstructure of ferrite and pearlite, and having a composition of, by weight:
carbon between 0.25 and 0.55%,
silicon between 0.15 and 0.35%,
manganese between 0.40 and 0.60%,
chromium between 0.80 and 1.10%,
molybdenum between 0.15 and 0.25%,
sulfur less than 0.040%,
phosphorus less than 0.035%;
(b) heating the provided steel composition to a temperature of 850-1150° C. (1562-2102° F.) in less than ten seconds;
(c) holding the heated steel composition at the peak temperature range for between two and sixty seconds;
(d) quenching the heated steel composition to below 100° C. in less than twenty seconds;
(e) removing residual quench media from the surface of the quenched steel composition by at least one of mechanical wiping, blown air, and combinations thereof;
(f) tempering the quenched steel composition at a temperature of; a range from 100° C. to 260° C. (212-500° F.) for less than 90 minutes;
(g) air cooling the tempered steel composition to less than 100° C. (212° F.) having a transformed microstructure of at least 80% martensite and up to 5% bainite, a yield strength of at least 1800 MPa, and a total elongation between 5% and 12%.
Again, the steel composition may be preheated at least 2.2° C./sec (36° F./sec) to not more than 815° C. (1500° F.) before heating in step (b). Alternatively, the steel composition again may be preheated to a temperature between 260° C. (500° F.) and 815° C. (1500° F.) and then preheated at least 0.7° C. (33.3° F.) per second to not more than 538° C. (1000° F.) before step (b). In another alternative, the steel composition may be preheated to a temperature between 260° C. (500° F.) and 815° C. (1500° F.) and preheating at least 0.7° C. (33.3° F.) per second to between the austenization temperature and 538° C. (1000° F.) before step (b).
Additionally, prior to heating the steel composition, two or more lengths of steel plates may be welded together along the width with one or more welds to form a continuous series of steel plates. Further, the step of welding may include applying a weave weld bridging between lengths of steel plate across the width of the steel plates. Further, the step of welding may include applying a weave weld bridging between lengths of steel plate in three sections where the center portion of steel plate is welded first and the side portions are welded to provide a weave weld across the width of the steel plates. In any event, a seam weld is applied over the weave weld across the width of the steel plates. Further, an indicia may be applied to the steel plate in advance of the welding step to enable a vision system to identify the location of end portions of lengths of the steel plates for the welding step.
During the quenching step, the heated steel composition again may be quenched from the peak temperature range to below 100° C. (212° F.) at a temperature rate reduction of between 400 and 3000° C./second (752-5432° F./sec). In the disclosed method, the quenching step may be performed by flowing a quench medium over the steel article at a rate of up to 900 gallons/min (3400 L/min). In one alternative, the quench medium may be water. Following quenching, the residual quench media may be removed from the surface of the quenched steel composition by at least one of mechanical wiping, blown air, and combinations thereof.
The quenched steel composition may be induction tempered for less than ten minutes, while in one alternative the quenched steel composition may be oven tempered for less than ninety minutes, and in another alternative the quenched steel composition may be tempered by a combination of oven and induction tempering for 30-60 minutes. The quenching step may be, for example, performed in more than 1 second and not more than 20 seconds. In still yet another alternative, the quenched steel composition may be induction tempered for two minutes or less. The tempering step may be performed at between 120° C. (250° F.) and 240° C. (500° F.). After quenching or tempering step, the steel plate may be cut into lengths at least at the seams to make substantially rectangular processed steel product while the steel plate continuously moves along the conveyor.